Marker Beacons Explained: Outer, Middle, Inner, and Back Course Markers, Compass Locators, the OM Altitude Cross-Check, and What 91.175(k) Says Can Replace Them
Long before GPS, glass cockpits, and moving maps, instrument pilots relied on a simple system to confirm their position on final approach: marker beacons. A 75 MHz transmitter on the ground, a light and a tone in the cockpit, and a definite “you are here” at a known point on the ILS. Most middle markers are gone, many outer markers have been decommissioned in favor of DME and GPS fixes, and inner markers are rare — but marker beacons remain on the instrument knowledge test, in every instrument oral, on legacy plates, and in the receivers of a large part of the training fleet. More importantly, the outer marker’s real job — a hard altitude cross-check on the glideslope — is a habit worth keeping even when the fix is a GPS waypoint instead of a blue light.
This post covers what marker beacons are and how they work, the four types (outer, middle, inner, and back course) with their tones, colors, keying, and locations, compass locators and their identifiers, the OM altitude cross-check, what 91.175(k) actually requires and what can substitute for the outer marker, why markers are being phased out, how to use the receiver, and the checkride questions.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
What a Marker Beacon Is
Position, not guidance.
The basics:
A ground-based, low-power transmitter on 75 MHz — all marker beacons use the same frequency, so there’s nothing to tune
Radiates a narrow fan-shaped (elliptical) pattern straight up
When you fly through the pattern, the cockpit receiver lights a colored lamp and plays a tone with a distinctive keying pattern
Tells you that you are passing a specific point on the approach — nothing more
What it does NOT do:
No lateral guidance (that’s the localizer)
No vertical guidance (that’s the glideslope)
No distance information (that’s DME or GPS)
No identification of which runway or airport — the color/tone identifies the marker type, and the chart tells you where it is
Why the pattern is narrow:
The fan is tight along the approach course so the indication is brief and the position fix is sharp
Crossing time at approach speed is a few seconds
The beam is wider across the course than along it, so being slightly off the localizer doesn’t make you miss it
Short indication = precise fix
The three cues:
Visual: the marker lamp (blue, amber, or white)
Aural: the tone at a specific pitch, keyed in a specific pattern
Positional: the chart tells you what the marker means (glideslope intercept check, decision-altitude vicinity, and so on)
The Four Marker Beacons (AIM 1-1-9)
The table examiners want, with the fourth entry most posts leave out.
Marker | Tone | Keying | Light | Location | Meaning |
Outer (OM) | 400 Hz (low) | Continuous dashes (— — —) | Blue | 4–7 NM from the threshold | Approximately where an aircraft at the published altitude on the localizer intercepts the glide path |
Middle (MM) | 1300 Hz (medium) | Alternating dots and dashes (· — · —) | Amber | About 3,500 feet from the threshold | Approximately where the glide path is about 200 feet above the touchdown zone elevation — roughly the CAT I decision altitude vicinity |
Inner (IM) | 3000 Hz (high) | Continuous dots (· · · ·) | White | Between the MM and the threshold | The point where an aircraft on the glide path is at the CAT II decision height |
Back Course (BCM) | 3000 Hz (high) | Pairs of dots (· · · ·) | White | On the localizer back course | Identifies the FAF or a fix on a localizer back course approach |
The pattern as a memory aid:
As you get closer to the runway, the pitch goes UP (400 → 1300 → 3000 Hz) and the keying gets FASTER and shorter (dashes → dots and dashes → dots)
Low, slow, far out; high, fast, close in
Colors: blue, amber, white from far to near
If the tone is rising and quickening, the runway is getting closer
Outer marker details:
Normally placed so that an aircraft at the glideslope intercept altitude on the localizer will cross it at the moment of intercept — which is why it’s the altitude cross-check point (below)
4 to 7 NM out
Often colocated with a compass locator (then it’s an LOM — below)
The marker that still matters most
Middle marker details:
At roughly 200 feet above TDZE on the glide path, it marks the CAT I DA vicinity
Before radio altimeters and DME were common, it gave the pilot a positive “you’re at minimums” cue
No longer a required ILS component (91.175(k) — below), and most have been decommissioned
You’ll rarely hear one today
Inner marker details:
Installed only where CAT II (and III) operations exist — major airports with low-visibility capability
Marks the CAT II DH point (around 100 feet)
Largely superseded by the radio altimeter as the CAT II/III DH reference
Rare
Back course marker details:
Only on localizer back course approaches, and only at some of them
Same 3000 Hz / white as the inner marker, but keyed in pairs of dots so you can tell them apart
Identifies the BC final approach fix or a fix on the BC final
The one most pilots forget exists
Compass Locators: LOM and LMM
The NDB that lives at the marker.
What they are:
A low-powered nondirectional beacon (NDB) colocated with a marker beacon site
At the outer marker: an LOM (Locator Outer Marker). At the middle marker: an LMM (Locator Middle Marker)
Power about 25 watts, range about 15 NM
Lets an ADF-equipped aircraft home to or track from the marker position — so the fix can be identified by ADF even without a marker receiver, and the approach can be flown from the LOM as an IAF
Identification:
The LOM identifier is the first two letters of the localizer identification group
The LMM identifier is the last two letters
Example: localizer I-ABC → LOM is “AB,” LMM is “BC”
Transmitted in Morse on the NDB frequency, not on 75 MHz
Why they matter:
An LOM can serve as an initial approach fix (the classic “hold at the LOM, procedure turn, fly the ILS” plate)
A compass locator is one of the authorized substitutes for the outer marker under 91.175(k)
On older ILS plates, the LOM frequency and identifier are in the plan view at the OM position
Fewer every year as ADF disappears, but still on charts
The OM Altitude Cross-Check
The marker beacon’s most important operational job — and a habit to keep at any FAF.
What you do:
As you cross the outer marker on the glideslope, read your altimeter and compare it to the published glideslope-crossing altitude at the OM (shown in the profile view, often with the lightning-bolt symbol)
They should agree closely — within about 50–100 feet is typical
If they don’t, something is wrong: a false glideslope, a wrong altimeter setting, or a glideslope receiver problem
One glance, every ILS
What it catches:
A false glideslope: the glideslope antenna radiates spurious lobes at steeper angles (roughly twice the published angle and up); if you intercepted one by descending onto the glideslope from above, you’d cross the OM far higher than published
A mis-set altimeter: a 1 inch error is about 1,000 feet, and the OM check exposes it
A glideslope receiver malfunction indicating “on glidepath” when you aren’t
The check is the reason the OM’s position was chosen
With no marker:
The same check applies at whatever defines the FAF/glideslope-intercept point — a DME fix, a GPS waypoint, or a cross-radial
The chart still publishes the crossing altitude at that fix
The light and tone were the cue; the altitude comparison is the point
Keep the habit
Markers and the ILS Approach Structure
Where the FAF really is.
On the ILS (precision):
The final approach fix is the glideslope intercept point at the published intercept altitude — charted with the lightning-bolt symbol in the profile view
The OM is normally located there, which is why “the OM is the FAF” is close to true — but the FAF is the glideslope intercept, and the OM is the position cue for it
Cross-check the altitude there
Then fly the glideslope to DA
On the localizer-only (nonprecision):
The FAF is the Maltese-cross symbol in the profile view, and it’s frequently placed at the OM (or at a DME/GPS fix where the OM has been decommissioned)
Start timing (if the MAP is time-defined) and the descent to the MDA at the FAF
The OM gives a positive crossing indication for both
A real operational fix on the LOC approach
Middle and inner markers:
Position cues in the DA/DH vicinity, not fixes you act on — you act on the altimeter at DA (or the radio altimeter at CAT II/III DH)
The MM was a “you should be at minimums about now” confirmation
Never a substitute for the DA
Confirmation, not decision
Reading the plate:
Marker positions appear in the plan view (the elliptical marker symbol) and profile view, with the glideslope-crossing altitude at the OM
An LOM shows an NDB symbol at the OM with its frequency and identifier
Decommissioned markers are simply gone from current plates; the FAF becomes a named fix
If the plate shows a marker and it’s NOTAMed out, you need a substitute (next section)
What the Regulation Requires: 91.175(k)
The basic components of an ILS, and what can replace the outer marker.
The basic components:
91.175(k): the basic components of an ILS are the localizer, the glide slope, and the outer marker — and, when installed for use with CAT II or III procedures, an inner marker
The middle marker is NOT a basic component — which is why MMs have been removed nationwide with no procedural impact
The outer marker IS — but it can be substituted
Know the list
Authorized substitutes for the outer marker:
A compass locator (the LOM)
Precision approach radar (PAR) or airport surveillance radar (ASR) — ATC can call the fix
DME, VOR, or NDB fixes authorized in the standard instrument approach procedure
A suitable RNAV system in conjunction with a fix identified in the approach procedure — i.e., GPS identifying the charted fix
Any one of these makes the OM’s absence a non-event
Inner marker substitution:
For CAT II/III, applicability of and substitution for the inner marker is determined by the Part 97 procedure, the operator’s letter of authorization, or operations specifications
In practice the radio altimeter DH has replaced it
A CAT II/III operator question, not a GA one
Covered by the operator’s authorization
Is a marker beacon receiver required equipment?
Not listed in 91.205(d) for IFR; 91.205(d) requires navigation equipment suitable for the route and approach to be flown
You can legally fly an ILS without a marker receiver if the OM (or its substitute) can be identified by other means — DME, GPS, ADF to the LOM, or radar
Most GA audio panels still include a marker receiver, so the question rarely arises
But it’s the correct answer on the oral
The practical rule:
OM out of service (or decommissioned) → identify the fix by DME, GPS, LOM, or radar as the plate and 91.175(k) allow
MM out of service → no effect
Localizer or glideslope out of service → that’s a different approach (LOC-only) or no approach
Read the NOTAMs and the plate notes
Why Marker Beacons Are Being Phased Out
The regulatory and practical story.
What replaced them:
DME fixes on the localizer (the localizer-DME, “I-ABC DME”)
GPS/RNAV waypoints that identify the same points — and 91.175(k) explicitly authorizes a suitable RNAV system as an OM substitute
Radar fixes from approach control
Radio altimeters for CAT II/III DH (replacing the IM’s function)
Why the FAA removed them:
Each marker is a separate site to lease, power, maintain, and flight-check, for a very small amount of information
91.175(k) allows substitutes for the OM and doesn’t require the MM at all, so removal has no procedural cost once DME or GPS fixes are published
Marker sites off the airport (the OM is 4–7 NM out) are exposed to land-use problems, vegetation, and vandalism
Everything a marker tells you, a DME or GPS fix tells you better
What you’ll see on plates:
Notes like “DME or RNAV required” when the OM has been decommissioned and the FAF/step-down fixes are DME/GPS-defined
Named GPS fixes where the OM used to be
Fewer LOMs as ADF fades
Occasionally a surviving OM or LOM at older ILS installations
What hasn’t changed:
The glideslope-intercept altitude cross-check
The FAF location
The decision-altitude logic
Markers were the cue; the procedure is the same
Using the Receiver
How the cockpit side works.
Controls:
A marker beacon audio switch (on/off or mute) on the audio panel — leave the audio on during an approach so you hear the tone even if you’re not looking at the light
A Hi/Lo sensitivity switch on many receivers: HIGH gives an earlier, longer indication; LOW gives a shorter indication closer to the actual overhead point and a more precise fix
Typical technique: HIGH to make sure you don’t miss it, LOW if you want the sharpest position fix
A test function that lights all three lamps and plays the tones
The lights:
Blue (OM), amber (MM), white (IM and BCM) — the color tells you which marker you’re crossing
Modern audio panels and glass displays show the same colors (or letter annunciations: O, M, I) and may route the tone through the headset
A momentary indication of a few seconds at approach speed
Watch and listen — the tone is often noticed first
Limitations:
Extremely short indication — at higher groundspeeds you can cross a marker in two or three seconds; a distracted pilot misses it
No indication of how far away it is until you’re on it, and nothing after you’ve passed it
Terrain, snow, or vegetation at the site can weaken the signal
A missed marker is not a missed fix if you have DME or GPS — cross-check the other source
Integration with the scan:
Approaching the OM: anticipate it by DME/GPS distance, be ready to read the altimeter
Crossing: light and tone → altimeter check → note the time if the MAP is timed
After: confirm the glideslope is being flown (or the descent to the MDA has begun)
Thirty seconds of attention, then back to the approach
Common Pilot Mistakes
Thinking a marker gives guidance or distance — it only confirms a point
Forgetting the back course marker exists (and confusing its pairs of dots with the inner marker’s continuous dots)
Skipping the OM altitude cross-check — the one thing the OM is for
Treating the middle marker as a required ILS component (it isn’t under 91.175(k))
Not knowing the OM substitutes (compass locator, PAR/ASR, DME/VOR/NDB fixes, RNAV)
Assuming the OM is always the FAF — on the ILS the FAF is the glideslope intercept (lightning bolt); on the LOC-only it’s the Maltese cross
Mixing up the LOM/LMM identifier rule (first two letters OM, last two MM)
Muting the marker audio and then not seeing the light
Missing the marker at high groundspeed and not falling back on DME/GPS
Confusing marker tone frequencies with the 75 MHz carrier (all markers transmit on 75 MHz; the tones are 400/1300/3000 Hz)
Checkride Questions — With Answers
“What frequency do marker beacons transmit on?”
75 MHz — all of them. The 400, 1300, and 3000 Hz figures are the audio tones, not the radio frequency.
“Describe the outer, middle, and inner markers.”
Outer: 400 Hz, continuous dashes, blue, 4–7 NM out, approximately at glideslope intercept. Middle: 1300 Hz, alternating dots and dashes, amber, about 3,500 feet from the threshold, about 200 feet above TDZE on the glide path. Inner: 3000 Hz, continuous dots, white, between the MM and the threshold, at the CAT II DH point.
“Is there a fourth marker?”
The back course marker: 3000 Hz, pairs of dots, white, on a localizer back course approach, marking the FAF or a fix on the BC final.
“What’s a compass locator, and how is it identified?”
A low-powered NDB colocated with a marker (LOM at the outer, LMM at the middle). The LOM identifier is the first two letters of the localizer ident; the LMM uses the last two.
“What are the basic components of an ILS?”
Per 91.175(k): the localizer, the glide slope, and the outer marker — plus an inner marker when installed for CAT II/III. The middle marker is not a basic component.
“The outer marker is out of service. Can you fly the ILS?”
Yes, with an authorized substitute: a compass locator, PAR or ASR, a DME/VOR/NDB fix authorized in the procedure, or a suitable RNAV system identifying a fix in the procedure (91.175(k)).
“What do you do when you cross the outer marker?”
Cross-check the altimeter against the published glideslope-crossing altitude at the OM — it catches a false glideslope or a mis-set altimeter. On a LOC-only approach, also start timing/descent at the FAF.
“Is a marker beacon receiver required for IFR?”
Not specifically under 91.205(d). You need navigation equipment suitable for the approach; if the OM or its substitute can be identified another way (DME, GPS, LOM, radar), a marker receiver isn’t required.
Final Thoughts
Marker beacons may be fading from operational use, but they’re still a foundational part of instrument knowledge — and the outer marker’s purpose outlives the hardware. A blue light and a low tone were never the point; the point was a positive position fix at the glideslope intercept where you check your altimeter against the chart before committing to the glideslope. That check still belongs on every ILS, whether the fix is an OM, an LOM, a DME distance, or a GPS waypoint.
Know the four markers, their tones and colors and where they sit, the compass locator identifier rule, what 91.175(k) calls a basic ILS component and what can stand in for the outer marker, and you’ll handle both the oral and the NOTAM that says “OM OTS” without a second thought.
On the Written Test and Checkride
Marker beacons appear on the instrument knowledge test and in instrument orals. The most commonly tested topics:
75 MHz carrier; OM/MM/IM tones (400/1300/3000 Hz), keying, colors, locations
The back course marker
Compass locators: LOM/LMM and the first-two/last-two identifier rule
91.175(k): basic ILS components; OM substitutes; MM not required
The OM glideslope-altitude cross-check and false glideslopes
FAF location on the ILS (glideslope intercept) vs. LOC-only (Maltese cross)
Marker receiver not specifically required under 91.205(d)
Quick Reference
Marker Beacons:
All on 75 MHz; narrow fan pattern straight up; a light + tone when you cross — position only, no guidance, no distance
Receiver: audio on during approaches; Hi (earlier/longer) / Lo (sharper fix) sensitivity; test button
The Four Markers (AIM 1-1-9):
Tone | Keying | Light | Where | Means | |
OM | 400 Hz | — — — dashes | Blue | 4–7 NM | ≈ glideslope intercept |
MM | 1300 Hz | · — · — | Amber | ~3,500 ft | ≈ 200 ft above TDZE (CAT I DA vicinity) |
IM | 3000 Hz | · · · · dots | White | MM → threshold | CAT II DH point |
BCM | 3000 Hz | · · · · pairs of dots | White | Localizer back course | BC FAF / fix |
Closer to the runway: pitch rises, keying speeds up; blue → amber → white
Compass Locators:
Low-power NDB (~25 W, ~15 NM) at a marker site: LOM (outer), LMM (middle)
Ident: LOM = first two letters of the localizer ident; LMM = last two
Can be an IAF; an authorized OM substitute
The OM Cross-Check:
Crossing the OM on the glideslope → compare altimeter to the published GS-crossing altitude
Catches false glideslopes (steeper spurious lobes), mis-set altimeters, GS receiver faults
Do the same at any FAF/GS-intercept fix — DME, GPS, or cross-radial
FAF Location:
ILS: glideslope intercept at the published altitude (lightning bolt); the OM is normally there as the cue
LOC-only: Maltese cross, often at the OM or its DME/GPS replacement; start timing/descent there
MM/IM: position cues near DA/DH, not fixes you act on
91.175(k):
Basic ILS components: localizer, glide slope, outer marker (+ inner marker for CAT II/III)
Middle marker is NOT required
OM substitutes: compass locator; PAR or ASR; DME/VOR/NDB fixes in the procedure; suitable RNAV system with a charted fix
IM substitution: per Part 97 procedure / LOA / ops specs
Marker receiver not specifically required by 91.205(d) if the fix can be identified otherwise
Phase-Out:
Replaced by localizer DME fixes, GPS/RNAV waypoints, radar fixes, radio altimeter DH
“DME or RNAV required” notes where OMs are gone; named fixes replace markers
The altitude cross-check and FAF logic are unchanged
Key Principle:
A marker beacon is a position cue, not guidance: 75 MHz, a color and a tone, four types (blue/400/dashes outer; amber/1300/dot-dash middle; white/3000/dots inner; white/3000/paired-dots back course). The outer marker’s job is the glideslope altitude cross-check at the FAF — keep doing it at whatever fix replaces it. Under 91.175(k) the OM is a basic ILS component that can be substituted by a compass locator, radar, DME/VOR/NDB fixes, or RNAV; the middle marker isn’t required at all.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.